1. Field of the Invention
The present invention relates to a process for producing a semiconductor device including an ion implantation step and, more particularly, to the process mentioned above suitable to be applied for the production of an MIS type FET, in which production an impurity concentration in the channel of the MIS type FET (hereinafter referred to as MISFET) is controlled by ion implantation.
2. Description of the Prior Art
Ion implantation is a standard technique for the surface doping of a semiconductor and comprises an introduction of atoms into the surface layer of a solid semiconductor substrate by bombardment of the semiconductor substrate with ions having an energy in the range of KeV to MeV. The previous diffusion method is being partly replaced by the ion implantation method, since the ion implantation method has the advantages of an externally controlled, non equilibrium process, whereas the diffusion process is dominated by unchangeable physical conditions, such as the diffusion constant of the impurity in the semiconductor material.
It is known in the ion implantation technique that the implanted atoms become electrically active only after heating the ion implanted, semiconductor material at a temperature of approximately 1000.degree. C., over a period of approximately 30 minutes. Damage to the atomic arrangement of the semiconductor material is also provoked by ion implantation, but can be remedied by such heating as mentioned above.
Since the ion implantation is carried out under a high energy, non equilibrium condition, the concentration of the implanted ion is maximum at the interior of the semiconductor which is referred to as the projected range (R.sub.p) or mean depth of penetration. The projected range (R.sub.p), and thus the impurity concentration, can be precisely varied by the energy of the ion beam. Accordingly, the ion implantation is suited for precisely controling a concentration of impurities having one conductivity type and to be contained in active regions of bipolar transistors, MISFETs and the like.
It is known in the field of semiconductor technology that the ions, frequently boron ions, are implanted through an insulative, thermally oxidized film on a semiconductor substrate under such an ion energy that the ions are implanted below the oxide film. An important purpose of applying the ion implantation mentioned above is a so called "channel doping", wherein the threshold voltage of the channel of MISFETs is adjusted by the ion implanted concentration in such channel. The known process of ion implantation was, however, disadvantageous because of the oxide film used as the insulation film, in the fact that the implanted boron ions are drawn out of the semiconductor substrate into the oxide film after the heating mentioned above, with the consequence that the impurity concentration at the surface of the semiconductor substrate is reduced to a level considerably lower than the required level. This is due to the fact that the distribution constant of the boron ions in the oxide film is high, and hence, a large amount of the boron ions are redistributed into the oxide film. Accordingly, it was not possible by means of the known channel doping process to obtain a desired value of threshold voltage.